Magnetic Mount vs NMO antenna issues

[JR4815D2]

I recently decided to install a NMO mount on my SUV. I was using a magnetic mount antenna prior to the NMO, which gave me great reception for about 25 to 30 miles. I mainly use it to listen to railroad channels. I am using Larsen 152-162 MHz VHF Whip NMO Antenna Model No. NMOQW152. Once, I installed the NMO mount, with the same antenna, reception drop to under 20 miles. I checked the cable to see if it was broken along the line, but line is good. I installed it in the same location as where the magnetic mount was placed. Could anyone share some info what went wrong? I am using both antennas mount with identical antennas. I could get much better reception with the magnetic mount. I waited to mount the NMO, now I feel like I shouldn't have done it. In my other vehicle I installed NMO mount and had no problems. It actually worked much better than the magnetic mount. I have a Ford Explorer and it is mounted towards the rare on the roof. Any info would be greatly appreciated.

 

[w2xq]

Assuming the antenna itself hasn't broken an internal winding, have you checked the cable with a VOM? A physical inspection may not disclose a broken shield or center conductor or a short between the shield and center conductor. Is there a good ground connection -- no paint -- for the mount itself on the underside of the roof? A VOM -- even an inexpensive one <$10-20 -- should help you diagnose the problem(s). HTH.

 

[nanZor]

Tom's right about doublechecking with a meter.

Make sure the tab on the NMO mount is actually touching the antenna inside the base. If you are using an O-ring or some other gasket between the mount and the antenna base, you may need to gently pull the center tab upwards a little bit. If your antenna isn't painted, you can run the continuity test again from the center of the coax connector across the antenna element itself.

 

[wnyf76]

Also you might want to check that part of the nmo that contacts the roof is grounding properly.

I always grind a little paint off around the hole to get a good ground.

 

[JR4815D2]

Also you might want to check that part of the nmo that contacts the roof is grounding properly.

I always grind a little paint off around the hole to get a good ground.

Thanks to everyone. I have done the continuity test with a multimeter and it comes back good. I also checked the ground by testing around the car and the mount. I also did the removal of paint in both sides enough for O-ring to cover it from the beginning. I sanded the area down to the metal. I order a new NMO mount. It should be here by the middle of the week. Maybe that would help.


NMO mount on roof top | Flickr - Photo Sharing!

 

[FFPM571]

Is that a used mount?

 

[w2xq]

Looking at the picture, is that oxidation on the center pin? Every few months I use a steel wood pad to clean the pin on the mount on my car and the center pin on the antenna. The gasket isn't perfect... but then I've had the mount on the car for 7 years...

 

[JR4815D2]

Is that a used mount?

I had it for a while in another vehicle. I just got a new one today.

Looking at the picture, is that oxidation on the center pin? Every few months I use a steel wood pad to clean the pin on the mount on my car and the center pin on the antenna. The gasket isn't perfect... but then I've had the mount on the car for 7 years...

I cleaned it a few times, the center has some sort of discoloration and I am not sure if that is affecting it. I will install the new one tonight.

 

[FFPM571]

As a rule my shop never reuses NMO mounts. There is too much chance of broken/ oxidized connectors or coax. Its cheap insurance. Is your connector end to the radio preconnected ? 50% of the time its a poor connection and causing a short. If you dont have it connected find someone who can put the end on for you.

 

[BirkenVogt]

The most common NMO mounts come with a very cheap grade of coax cable that is lossy. It is not usually a big problem on high band, though. Just something to consider.

If it still has poor performance I would take it to a radio shop or a ham who has an antenna analyzer and check the SWR of the mount plus antenna on 161 MHz or wherever you are listening. Also they make an adapter from NMO to PL-259 so you can check the connection into a dummy load to see that the cable is at least good before adding an antenna. Without these tests you are basically shooting in the dark.

 

[KOK5CY]

I never drill holes for roof mount's ...I'd rather use mag mnt's and put it either on the top or trunk lid ...But my set up is a nmo150 with a 49in whip and can hear anything out there ...vhf hi , uhf ..just sayin

 

[LtDoc]

Nothing new in this post, basically the same things that have already been said. There has to be a problem with some 'part' of the antenna system, the mount, the feed line, or the connector on the end of that feed line, for there to be a reduction in what you are hearing/not hearing. Simply changing from a mag-mount to a fixed mount shouldn't make that much difference with the particular antenna using those mounts.
Checking continuity between all those parts is certainly a good thing to do. From there, it's get's more complicated because you are looking for the 'why' behind it happening. Most of the times I've had problems it's been because I usually goofed in some way, and it's usually something that falls in the, "I know better than that!", category. And of course there's the fact that you can always get a 'lemon' no matter what you're talking about.
Have fun.
- 'Doc